This invention relates to a method and apparatus for the aseptic packaging of high acid food products. More specifically, the present invention is directed to a method whereby preformed barrier plastic containers are immersed in an aqueous sterilizing bath at a temperature ranging from 212.degree. F. to 140.degree. F. for a time interval ranging from less than 1 to 60 seconds, following which, after draining the sterilizing liquid therefrom, the containers are filled with pre-sterilized high acid food product under essentially inert atmosphere conditions and thereafter closed in an inert atmosphere by sealing a sterile closure element thereto. Novel apparatus for automatically carrying out the method of the invention is also provided.
Although packaging of low pH food products has been conducted in glass bottles and metal cans for many years, costs of these containers, their closures and labels have increased rapidly. Cost of thermoplastic materials has not increased as rapidly and are also more conserving of energy on a systems basis. Therefore, packaging in thermoplastic materials represents a significant savings in packaging costs. Because of the heat sensitivity of most thermoplastics, however, conventional hot filling and terminal thermal sterilization techniques damages or destroys such packages. On the other hand, application of aseptic techniques would permit sterilization of the packages under controlled conditions which leave the packaging unaltered. As a further advantage of aseptic packaging, the product is heated and cooled rapidly prior to packaging, and so is minimally altered by processing. A higher quality product results from the practice of this invention.
The high acid food products contemplated are those juices, fruits, vegetables and products having a pH less than 4.6. Included are fruit juices, fruit juice drinks, fruit flavored drinks, nectars, sauces, and purees in fluid and semifluid form, and similar low pH dairy products such as fermented milk products, yoghurt, jam, jellies, fruit syrups, vinegar, catsup, tomato juice and gelatin desserts. Also contemplated are food products with a water activity greater than 0.85 and pH less than 4.6 such as whole, sliced and diced fruit, tomato products and formulated food products.
Microorganisms within normal tissues of fruits and related agricultural commodities most frequently are gram negative motile rods, representative of the Pseudomonadacea and the Enterobacteriaceae species. It appears that the bacteria can enter living plant tissue by various pathways and persist there as harmless commensals.
The surface flora of fruits, however, are quite different with lactic acid forming bacteria such as Lactobacilli and Leuconostoc being common. On apples, during harvest, yeasts dominate, but occasionally acetic bacteria are present in large numbers. Typically, 10.sup.3 to 10.sup.4 microorganisms per square centimeter epiderm are observed on healthy apples. These numbers rapidly increase to 10.sup.6 to 10.sup.7. In diseased fruits as many as 10.sup.8 microorganisms have been found on the surfaces of oranges. Thus, microorganisms are natural and normal flora in and on fruits, and serve as a reservoir to contaminate fruit products when they are processed. For example, 54 different yeasts have been isolated from unpasteurized citrus fruit juices and concentrates in typical citrus juice processing plants. The most common genera were Candida, Pichia, Zygosaccharomyces, Saccharomyces, and Hanseniaspora. Additionally, five genera of bacteria have been found in unpasteurized citrus juices, Lactobacillus, Leuconostoc, Aerobacter, Xanthomonas, and Achromobacter. An average of 10.sup.5 to 10.sup.7 microorganisms per milliliter are found in freshly extracted citrus juices.
The conspicuous characteristic distinguishing sterilization of fruit products from most other foods is their low pH which generally decreases the heat resistance of microorganisms, and is the reason that thermal sterilization of fruit products can be accomplished by heating for less than five minutes below 212.degree. F. Yeasts are readily killed in acid food products by mild heat treatment. Yeast cell destruction can be accomplished by heating for a few seconds to a few minutes at 160.degree. F., and their spores are destroyed at only a few degrees higher. As a rule, yeast cell destruction by heat is not greatly affected by pH. The non-sporeforming aciduric bacteria are quite similar to yeasts with respect to heat lability, but their destruction is greatly enhanced as pH decreases.
Bacterial endospores are heat resistant, but their thermal destruction also occurs readily at high hydrogen ion concentrations, i.e., low pH. At pH 4, the heat susceptability of the aciduric sporeformers is reduced so far that destruction occurs with a five minute heat treatment of 190.degree. F. Further, these sporeforming bacteria generally do not grow below pH 4. It is important to note that the only microorganisms of concern with high acid foods are those which can survive and grow at a pH of 4.6 or lower, a category which excludes pathogenic spore forming and other pathogenic bacteria.
In accordance with the present invention, the high acid food product to be packaged aseptically is processed into the form (e.g., juice) to be packaged, and is sterilized prior to packaging by suitable means, as by heating. Thus the food product delivered to the aseptic packaging operation is essentially free of spoilage microorganisms. The preformed plastic containers into which the sterile food product is to be packaged require sterilization before the food is placed therein. These containers and closures may be fabricated at a location remote from the food packaging operation, and while fabrication thereof is conducted under conditions of good manufacturing practice, opportunities exist for the contamination thereof by undesirable microorganisms which could cause spoilage of any food packed therein.
Another essential element of this invention is the reduction in oxygen in both the product contained and in the interior of the package, and the retention of this oxygen level below one percent of the headspace volume in the sealed package. High acid food products such as fruits and fruit products are susceptible to rapid loss of vitamin C, flavor and color by exposure to excessive levels of oxygen. In conventional canning practice, product is heated and filled hot prior to closure. Upon cooling, a partial vacuum of up to 22 in. Hg is effected through condensation of head space steam. Products contained in cans and bottles thus packaged undergo oxidative biochemical changes due to the presence of occluded and dissolved oxygen in the product and of residual oxygen in the headspace. The quantities present in conventional practice are sufficient to oxidize all the vitamin C naturally present and to render the product unacceptable.
In this invention, the product is treated prior to packaging to reduce the oxygen dissolved and occluded in the product. This invention mandates that the filling and closure be conducted in an inert atmosphere such as sterile nitrogen to insure that headspace oxygen after sealing is well below 1%. Further, this invention mandates that the packaging materials be composed of oxygen-barrier materials to minimize the quantity of oxygen that can migrate through the walls of the package after closure.